Question

Describe method of preparation of soap giving chemical equation for the reaction involved.

Answer 1

Saponification, or the hydrolysis of fats and oils in the presence of a strong base such as NaOH or KOH, is used to make soap. The general formula for Saponification is:

Triglyceride (fat or oil) + 3NaOH → Soap + Glycerol
Taking the saponification of a triglyceride found in vegetable oil as an example:

\[\begin{array}{cc} \ce{O}\phantom{......}\\ ||\phantom{......}\\ \ce{CH2 - O - C - (CH2)14 - CH3}\\ |\phantom{..........}\ce{O}\phantom{..................}\\ \phantom{..........}||\phantom{..................}\\ \phantom{....}\ce{CH - O - C - (CH2)14 - CH3 ->}\\ |\phantom{.............................}\\ \ce{CH2 - O - C - (CH2)14 - CH3}\\ ||\phantom{......}\\ \ce{O}\phantom{......}\\ \end{array}\]

Triglyceride

 

\[\begin{array}{cc} \phantom{}\ce{CH2 - OH}\phantom{...........}\ce{O}\phantom{.................}\\ \phantom{}|\phantom{.....................}||\phantom{.................}\\ \phantom{}\ce{CH - OH\phantom{}\underset{Sodium palrnitate (Soap)}{+ 3N\overset{+}{a}\overset{-}{O} - C - (CH2)14 - CH3}}\phantom{}\\ \phantom{}|\phantom{.......................................}\\ \phantom{}\ce{\underset{Glycerol}{CH2 - OH}}\phantom{..............................}\\ \end{array}\]

In this equation:

• Glycerol is a trihydric alcohol produced as a byproduct.
• The soap formed is a sodium salt of the fatty acid and it consists of a hydrophobic (water-repelling) tail and a hydrophilic (water-attracting) head.
• The triglyceride represents the fat or oil, which is a triesfer of glycerol and three fatty acids.
• Sodium hydroxide (NaOH) is the strong base used in the saponification process.

Soap molecules are amphiphilic, meaning they interact with both water and oil to lift dirt and grease from surfaces.